1. Field of the Invention
This invention relates generally to microwave amplifiers and, more particularly to a microwave amplifier which converts the amplified signals to an unoccupied television channel; e.g. channels 2 to 6 in the low range TV band (58 to 78 Mhz). The overall microwave energy pickup is increased by integrating active components into a truncated quarter spherical wave reflector which feeds the open end of a resonant RF cavity.
2. Description of the Prior Art
Circuits for tuning, transferring and coupling signals in receiving systems operating above 500 Mhz have usually utilized passive elements with distributed constants such as resonant lines or cavities. Lumped parameter networks for such frequencies are usually unsatisfactory because the small values of "L" and "C" vary with environmental changes thus seriously altering the resonant characteristics. Before the advent of microwave transistors, active receiving elements comprised triodes generally connected in a grounded grid configuration. Traveling wave tubes for RF amplification at microwave frequencies and crystal mixers with positive grid or transit time oscillators for frequency conversion have also been utilized in receiving systems. Wave guides, cavity resonators and associated plumbing exhibit low losses with reasonable stability, but at 2-4 Ghz, such systems are bulky and expensive.
A stripline comprises a shaped metallic foil cemented to a dielectric substrate which in turn is cemented to an underlying solid foil or ground plane. Strip lines have been used for many years but they are difficult to integrate with active thermionic devices, and when etched on an ordinary dielectric constant substrate, they are almost as large as the waveguides they replace.
Foil coated high dielectric substrates have recently become available. These low loss substrates are plastic loaded with a titinate ceramic which has a high dielectric constant. This permits a reduction in strip line dimensions by a factor of three or four for comparable frequencies. The commercial availability of low loss printed circuit boards and good performing relatively inexpensive microwave transistors render it feasible to produce amplifiers and converters which will operate satisfactorily as high as 40 Ghz. Thus, microwave systems can now be built for the consumer market.